1. Field of the Invention
The present invention relates to a method for producing a magnetic recording medium, and particularly to a method for producing a low-noise and high-density coating-type magnetic recording medium.
2. Description of the Related Art
For increasing the density of the magnetic recording medium, the method of increasing the tape length (reducing the tape thickness) to increase the volume recording density, and the method of reducing the track width to increase the area recording density have been generally used in many systems as for tape-shaped media.
Consequently, the reduced tape thickness has caused the problem of insufficient edge strength of tapes, thus bringing about a situation in which substrates of materials that have higher strength and are more expensive than conventional products must be used. Also, a servo technique has been developed for accommodating the recently adopted method of reducing the track width, but this technique is difficult to apply particularly in a linear-type system, and the cost of development is considerably be increased.
Methods other than those described above include the method of increasing the linear recording density as a method for increasing the density, but this method has been avoided because of the considerable reduction in C/N (carrier output/noise ratio) associated with short-wave recording.
Recently, an increasing number of systems have adopted high-sensitivity MR heads as playback heads as a method for increasing the density. In this case, reduction of the medium noise and extreme reduction in thickness of the magnetic layer are required for the magnetic recording medium. It can be considered that a finely granulated and uniformly distributed magnetic material is used, and the magnetic material is uniformly dispersed in preparation of a liquid as the method for reducing the medium noise.
In the conventional liquid preparation method, however, the initial contact of the magnetic material with a binder is achieved by a kneader or the like. In this case, since the coagulated magnetic material powder and the solution containing a high concentration of binder are mixed with a high shear force applied thereto, there arise problems such that the wettability of the magnetic material is reduced and hence the dispersibility of the magnetic material is compromised, and the coagulation of the magnetic material formed by kneading cannot be dispersed after it is dispersed, if a magnetic material of fine particles is used. Also, there is a problem that the coagulation of the magnetic material causes not only a noise source but also defects such as seams in coating of an extremely thin magnetic layer.